Mechanical actuation by responsive polyelectrolyte brushes and triblock gels

Progress in the development of actuating molecular devices based on responsive polymers is reviewed. The synthesis and characterization of "grafted from brushes and triblock copolymers is reported. The responsive nature of polyelectrolyte brushes, grown by surface initiated atomic transfer radical polymerization (ATRP), has been characterized by scanning force microscopy, neutron reflectometry, and single molecule force measurements. The molecular response is measured directly for the brushes in terms of both the brush height and composition and the force generated by a single molecule. Triblock copolymers, based on hydrophobic end blocks and polyacid midblock, have been used to produce polymer gels where the deformation of the molecules can be followed directly by small angle Xray scattering (SAXS), and a correlation between molecular shape change and macroscopic deformation has been established. A Landolt pHoscillator, based on bromate/sulfite/ferrocyanide, with a room temperature period of 20 min and a range of 3.1 polyelectrolyte molecules scale affinely to produce the macroscopic response of the system in an oscillating chemical reaction.

Strengthening mechanisms in nanostructured copper/304 stainless steel multilayers

Nanostructured Cu/304 stainless steel (SS) multilayers were prepared by magnetron sputtering. 304SS has a face-centered-cubic (fcc) structure in bulk. However, in the Cu/304SS multilayers, the 304SS layers exhibit the fcc structure for layer thickness of =5 nm in epitaxy with the neighboring fcc Cu. For 304SS layer thickness larger than 5 nm, body-centered-cubic (bcc) 304SS grains grow on top of the initial 5 nm fcc SS with the Kurdjumov-Sachs orientation relationship between bcc and fcc SS grains. The maximum hardness of Cu/304SS multilayers is about 5.5 GPa (factor of two enhancement compared to rule-of-mixtures hardness) at a layer thickness of 5 nm. Below 5 nm, hardness decreases with decreasing layer thickness. The peak hardness of fcc/fcc Cu/304SS multilayer is greater than that of Cu/Ni, even though the lattice-parameter mismatch between Cu and Ni is five times greater than that between Cu and 304SS. This result may primarily be attributed to the higher interface barrier stress for single-dislocation transmission across the {111} twinned interfaces in Cu/304SS as compared to the {100} interfaces in Cu/Ni.

The relationship between charge density and polyelectrolyte brush profile using simultaneous neutron reflectivity and in situ attenuated total internal reflection FTIR

We report on a novel experimental study of a pH-responsive polyelectrolyte brush at the silicon/D2O interface. A poly[2-(diethylamino)ethyl methacrylate] brush was grown on a large silicon crystal which acted as both a substrate for a neutron reflectivity solid/liquid experiment but also as an FTIR-ATR spectroscopy crystal. This arrangement has allowed for both neutron reflectivities and FTIR spectroscopic information to be measured in parallel. The chosen polybase brush shows strong IR bands which can be assigned to the N-D+ stretch, D2O, and a carbonyl group. From such FTIR data, we are able to closely monitor the degree of protonation along the polymer chain as well as revealing information concerning the D2O concentration at the interface. The neutron reflectivity data allows us to determine the physical brush profile normal to the solid/liquid interface along with the corresponding degree of hydration. This combined approach makes it possible to quantify the charge on a polymer brush alongside the morphology adopted by the polymer chains. © 2013 American Chemical Society.

PREPARATION OF POLYELECTROLYTE MEMBRANES EMBEDDED WITH ZEOLITE NANOPARTICLES FOR ENHANCED PERFORMANCE IN FORWARD OSMOSIS

Water scarcity is a global issue that has already affected every continent. Membrane technology is considered as one of the most promising candidates for resolving this worsening issue. Among all the membrane processes, the emerging forward osmosis (FO) membrane process is osmotically-driven and has unique advantages compared with other traditional pressure-driven membrane processes. One of the major challenges to advancing the FO membrane process is the lack of a suitable membrane. Polyelectrolyte thin film prepared via layer-by-layer (LbL) technique has demonstrated its excellent performance in many applications including electronics, optics, sensors, etc. Recent studies have revealed the potential of polyelectrolyte thin films in acting as the active separation layer of FO membranes, but significant efforts are still needed to improve the membrane performance and understand the transport mechanisms. This dissertation introduces a novel approach to prepare a zeolite-embedded polyelectrolyte composite membrane for enhanced FO performance. This membrane takes advantages of the versatile LbL process to unprecedentedly incorporate high loading of zeolite nanoparticles, which are anticipated to facilitate water transport due to the uniquely interconnected structure of zeolites. Major topics discussed in this dissertation include: (1) the synthesis and evaluation of the polyelectrolyte-zeolite composite FO membrane, (2) the examination of the fouling resistance to identify its technical limitations, (3) the demonstration of the membrane regenerability as an effective strategy for membrane fouling control, and (4) the investigation of crosslinking effects on the membrane performance to elucidate the transport mechanisms involved in the zeolite-embedded polyelectrolyte membranes. Comparative studies have been made between polyelectrolyte membranes with and without zeolite incorporation. The findings suggest that the zeolite-embedded membrane, although slightly more susceptible to silica scaling, has demonstrated enhanced water flux and separation capability, good resistance to organic fouling, and complete regenerability for fouling control. Additionally, the embedded zeolite nanoparticles are proved to be able to create fast pathways for water transport. Overall, this work provides a novel strategy to create zeolite-polymer composite membranes with enhanced separation performance and unique fouling mitigation properties.

Advanced Adhesion Strength Testing Methods of Thin Film Multilayers in Electronic Packaging Systems

With the continued miniaturization and increasing performance of electronic devices, new technical challenges have arisen. One such issue is delamination occurring at critical interfaces inside the device. This major reliability issue can occur during the manufacturing process or during normal use of the device. Proper evaluation of the adhesion strength of critical interfaces early in the product development cycle can help reduce reliability issues and time-to-market of the product. However, conventional adhesion strength testing is inherently limited in the face of package miniaturization, which brings about further technical challenges to quantify design integrity and reliability. Although there are many different interfaces in today's advanced electronic packages, they can be generalized into two main categories: 1) rigid to rigid connections with a thin flexible polymeric layer in between, or 2) a thin film membrane on a rigid structure. Knowing that every technique has its own advantages and disadvantages, multiple testing methods must be enhanced and developed to be able to accommodate all the interfaces encountered for emerging electronic packaging technologies. For evaluating the adhesion strength of high adhesion strength interfaces in thin multilayer structures a novel adhesion test configuration called “single cantilever adhesion test (SCAT)” is proposed and implemented for an epoxy molding compound (EMC) and photo solder resist (PSR) interface. The test method is then shown to be capable of comparing and selecting the stronger of two potential EMC/PSR material sets. Additionally, a theoretical approach for establishing the applicable testing domain for a four-point bending test method was presented. For evaluating polymeric films on rigid substrates, major testing challenges are encountered for reducing testing scatter and for factoring in the potentially degrading effect of environmental conditioning on the material properties of the film. An advanced blister test with predefined area test method was developed that considers an elasto-plastic analytical solution and implemented for a conformal coating used to prevent tin whisker growth. The advanced blister testing with predefined area test method was then extended by employing a numerical method for evaluating the adhesion strength when the polymer’s film properties are unknown.

Charged pullulan derivatives for the development of nanocarriers by polyelectrolyte complexation

Pullulan, a neutral polysaccharide, was chemically modified in order to obtain two charged derivatives: reaction with SO3(.)DMF complex afforded a sulfate derivative (SP), while reaction with glycidyltrimethylammonium chloride gave a quaternary ammonium salt (AP). The presence of the charged groups was confirmed by FTIR. Assessment of the positions where the reaction took place was based on (1)H- and (13)C NMR (COSY, HSQC-TOCSY, HSQC-DEPT, and HMBC) experiments. Estimation of the degree of substitution (DS) was made from elemental analysis data, and further confirmed by NMR peak areas in the case of AP. These new derivatives showed the capability to condense with each other, forming nanoparticles with the ability to associate a model protein (BSA) and displaying adequate size for drug delivery applications, therefore making them good candidates for the production of pullulan-based nanocarriers by polyelectrolyte complexation.

Superactivation of metal electrode surfaces and its relevance to COads oxidation at fuel cell anodes

The inhibiting effect of COads on platinum-based anodes is a major problem in the development of ambient temperature, polyelectrolyte membrane-type fuel cells. One of the unusual features of the response for the oxidative removal of the species in question is that the response observed for this reaction in the positive sweep is highly dependent on the CO admission potential, for example, when the COads is formed in the Hads region it undergoes oxidation at unusually low potentials. Such behaviour is attributed here to hydrogen activation of the platinum surface, with the result that oxide mediators (and COads oxidation) occurs at an earlier stage of the positive sweep. It is also demonstrated, for both platinum and gold in acid solution, that dramatic premonolayer oxidation responses may be observed following suitable preactivation of the electrode surfaces. It is suggested that the defect state of a solid electrode surface is an important variable whose investigation may yield improved fuel cell anode performance.

Magnetic nanoparticles boosting the osmotic efficiency of a polymeric FO draw agent: Effect of polymer conformation

Poly sodium acrylate (PSA)-coated Magnetic Nanoparticles (PSA-MNPs) were synthesized as smart osmotic draw agent (SMDA) for water desalination by forward osmosis (FO) process. The PSA-coated MNPs demonstrated significantly higher osmotic pressure (~ 30 fold) as well as high FO water flux (~ 2–3 fold) when compared to their polymer (polyelectrolyte) counterpart, even at a very low concentration of ~ 0.13 wt.% in the draw solution. The PSA polymer chain conformation – coiled to extended – demonstrates a significant impact on the availability of the polymer hydrophilic groups in solution which is the driving force to attain higher osmotic pressure and water flux. When an optimum concentration of the polymer was anchored to a NP surface, the polymer chains assume an extended open conformation making the functional hydrophilic groups available to attract water molecules. This in turn boosts the osmotic pressure and FO water flux of the PSA-MNP draw agents. The low concentration of the PSA-MNP osmotic agent and the associated high water flux enhances the cost-effectiveness of our proposed SMDA system. In addition, easier magnetic separation and regeneration of the SMDA also improves its usability making it efficient, cost-effective and environment-friendly.

Optimized multilayer indium-free electrodes for organic photovoltaics

Flexible multilayer electrodes that combine high transparency, high conductivity, and efficient charge extraction have been deposited, characterised and used as the anode in organic solar cells. The anode consists of an AZO/Ag/AZO stack plus a very thin oxide interlayer whose ionization potential is fine-tuned by manipulating its gap state density to optimise charge transfer with the bulk heterojunction active layer consisting of poly(n-3- hexylthiophene-2,5-diyl) and phenyl-C61-butyric acid methyl ester (P3HT:BC61BM). The deposition method for the stack was compatible with the low temperatures required for polymer substrates. Optimisation of the electrode stack was achieved by modelling the optical and electrical properties of the device and a power conversion efficiency of 2.9% under AM1.5 illumination compared to 3.0% with an ITO-only anode and 3.5% for an ITO:PEDOT electrode. Dark I-V reverse bias characteristics indicate very low densities of occupied buffer states close to the HOMO level of the hole conductor, despite observed ionization potential being high enough. Their elimination should raise efficiency to that with ITO:PEDOT.

Synthesis of novel poly[(2,5-dimethoxy-p-phenylene)vinylene] precursors having two eliminatable groups: An approach for the control of conjugation length

Poly[(2,5-dimethoxy-p-phenylene)vinylene] (DMPPV) of varying conjugation length was synthesized by selective elimination of organic soluble precursor polymers that contained two eliminatable groups, namely, methoxy and acetate groups. These precursor copolymers were in turn synthesized by competitive nucleophilic substitution of the sulfonium polyelectrolyte precursor (generated by the standard Wessling route) using methanol and sodium acetate in acetic acid. The composition of the precursor copolymer, in terms of the relative amounts of methoxy and acetate groups, was controlled by varying the composition of the reaction mixture during nucleophilic substitution. Thermal elimination of these precursor copolymers at 250 degrees C, yielded partially conjugated polymers, whose color varied from light yellow to deep red. FT-IR studies confirmed that, while essentially all the acetate groups were eliminated, the methoxy groups were intact and caused the interruption in conjugation. Preliminary photoluminescence studies of the partially eliminated DMPPV samples showed a gradual shift in the emission maximum from 498 to 598 nm with increasing conjugation lengths, suggesting that the color of LED devices fabricated from such polymers can, in principle, be fine-tuned.

Engineering cellulose nanofibre suspensions to control filtration resistance and sheet permeability

This study examines and quantifies the effect of adding polyelectrolytes to cellulose nanofibre suspensions on the gel point of cellulose nanofibre suspensions, which is the lowest solids concentration at which the suspension forms a continuous network. The lower the gel point, the faster the drainage time to produce a sheet and the higher the porosity of the final sheet formed. Two new techniques were designed to measure the dynamic compressibility and the drainability of nanocellulose–polyelectrolyte suspensions. We developed a master curve which showed that the independent variable controlling the behaviour of nanocellulose suspensions and its composite is the structure of the flocculated suspension which is best quantified as the gel point. This was independent of the type of polyelectrolyte used. At an addition level of 2 mg/g of nanofibre, a reduction in gel point over 50 % was achieved using either a high molecular weight (13 MDa) linear cationic polyacrylamide (CPAM, 40 % charge), a dendrimer polyethylenimine of high molecular weight of 750,000 Da (HPEI) or even a low molecular weight of 2000 Da (LPEI). There was no significant difference in the minimum gel point achieved, despite the difference in polyelectrolyte morphology and molecular weight. In this paper, we show that the gel point controls the flow through the fibre suspension, even when comparing fibre suspensions with solids content above the gel point. A lower gel point makes it easier for water to drain through the fibre network,reducing the pressure required to achieve a given dewatering rate and reducing the filtering time required to form a wet laid sheet. We further show that the lower gel point partially controls the structure of the wet laid sheet after it is dried. Halving the gel point increased the air permeability of the dry sheet by 37, 46 and 25 %, when using CPAM, HPEI and LPEI, respectively. The resistance to liquid flow was reduced by 74 and 90 %, when using CPAM and LPEI. Analysing the paper formed shows that sheet forming process and final sheet properties can be engineered and controlled by adding polyelectrolytes to the nanofibre suspension.

Self-Assembly of Bile Steroid Analogues: Molecules, Fibers, and Networks

Structural and rheological features of a series of molecular hydrogels formed by synthetic bile salt analogues have been scrutinized. Among seven gelators, two are neutral compounds, while the others are cationic systems among which one is a tripodal steroid derivative. Despite the fact that the chemical structures are closely related, the variety of physical characteristics is extremely large in the structures of the connected fibers (either plain cylinders or ribbons), in the dynamical modes for stress relaxation of the associated SAFINs, in the scaling laws of the shear elasticity (typical of either cellular solids or fractal floc-like assemblies), in the micron-scale texture and the distribution of ordered domains (spherulites, crystallites) embedded in a random mesh, in the type of nodal zones (either crystalline-like, fiber entanglements, or bundles), in the evolution of the distribution and morphology of fibers and nodes, and in the sensitivity to added salt. SANS appears to be a suitable technique to infer all geometrical parameters defining the fibers, their interaction modes, and the volume fraction of nodes in a SAFIN. The tripodal system is particularly singular in the series and exhibits viscosity overshoots at the startup of shear flows, an “umbrella-like” molecular packing mode involving three molecules per cross section of fiber, and scattering correlation peaks revealing the ordering and overlap of 1d self-assembled polyelectrolyte species.

Borax Mediated Layer-by-Layer Self-Assembly of Neutral Poly(vinyl alcohol) and Chitosan

We report a multilayer film of poly(vinyl alcohol) (PVA)-borate complex and chitosan by using a layer-by-layer approach. PVA is an uncharged polymer, but hydroxyl functional groups of PVA can be crosslinked by using borax as a cross-linking agent. As a result electrostatic charges and intra- and interchain cross-links are introduced in the PVA chain and provide physically cross-linked networks. The PVA-borate was then deposited on a flat Substrate as well as on colloidal particles with chitosan as an oppositely charged polyelectrolyte. Quartz crystal microbalance. scanning electron microscopy, and atomic force microscopy were used to follow the growth of thin film oil flat substrate. Analogous experiments were performed on melamine formaldehyde colloidal particles (3-3.5 mu m) to quantify the process for the preparation of hollow rnicrocapsules. Removal of the core in 0.1 N HCI results in hollow microcapsules. Characterization of microcapsules by transmission electron microscopy revealed formation of stable microcapsules. Further, self-assembly of PVA-borate/chitosan was loaded with the anticancer drug doxorubicin, and release rates were determined at different pH Values to highlight the drug delivery potential of this system.

Asymmetrical flow field-flow fractionation in the study of water-soluble macromolecules

Asymmetrical flow field-flow fractionation (AsFlFFF) was constructed, and its applicability to industrial, biochemical, and pharmaceutical applications was studied. The effect of several parameters, such as pH, ionic strength, temperature and the reactants mixing ratios on the particle sizes, molar masses, and the formation of aggregates of macromolecules was determined by AsFlFFF. In the case of industrial application AsFlFFF proved to be a valuable tool in the characterization of the hydrodynamic particle sizes, molar masses and phase transition behavior of various poly(N-isopropylacrylamide) (PNIPAM) polymers as a function of viscosity and phase transition temperatures. The effect of sodium chloride salt and the molar ratio of cationic and anionic polyelectrolytes on the hydrodynamic particle sizes of poly (methacryloxyethyl trimethylammonium chloride) and poly (ethylene oxide)-block-poly (sodium methacrylate) and their complexes were studied. The particle sizes of PNIPAM polymers, and polyelectrolyte complexes measured by AsFlFFF were in agreement with those obtained by dynamic light scattering. The molar masses of PNIPAM polymers obtained by AsFlFFF and size exclusion chromatography agreed also well. In addition, AsFlFFF proved to be a practical technique in thermo responsive behavior studies of polymers at temperatures up to about 50 oC. The suitability of AsFlFFF for biological, biomedical, and pharmaceutical applications was proved, upon studying the lipid-protein/peptide interactions, and the stability of liposomes at different temperatures. AsFlFFF was applied to the studies on the hydrophobic and electrostatic interactions between cytochrome c (a basic peripheral protein) and anionic lipid, and oleic acid, and sodium dodecyl sulphate surfactant. A miniaturized AsFlFFF constructed in this study was exploited in the elucidation of the effect of copper (II), pH, ionic strength, and vortexing on the particle sizes of low-density lipoproteins.

Polymer Protected Gold Nanoparticles

Polymer protected gold nanoparticles have successfully been synthesized by both "grafting-from" and "grafting-to" techniques. The synthesis methods of the gold particles were systematically studied. Two chemically different homopolymers were used to protect gold particles: thermo-responsive poly(N-isopropylacrylamide), PNIPAM, and polystyrene, PS. Both polymers were synthesized by using a controlled/living radical polymerization process, reversible addition-fragmentation chain transfer (RAFT) polymerization, to obtain monodisperse polymers of various molar masses and carrying dithiobenzoate end groups. Hence, particles protected either with PNIPAM, PNIPAM-AuNPs, or with a mixture of two polymers, PNIPAM/PS-AuNPs (i.e., amphiphilic gold nanoparticles), were prepared. The particles contain monodisperse polymer shells, though the cores are somewhat polydisperse. Aqueous PNIPAM-AuNPs prepared using a "grafting-from" technique, show thermo-responsive properties derived from the tethered PNIPAM chains. For PNIPAM-AuNPs prepared using a "grafting-to" technique, two-phase transitions of PNIPAM were observed in the microcalorimetric studies of the aqueous solutions. The first transition with a sharp and narrow endothermic peak occurs at lower temperature, and the second one with a broader peak at higher temperature. In the first transition PNIPAM segments show much higher cooperativity than in the second one. The observations are tentatively rationalized by assuming that the PNIPAM brush can be subdivided into two zones, an inner and an outer one. In the inner zone, the PNIPAM segments are close to the gold surface, densely packed, less hydrated, and undergo the first transition. In the outer zone, on the other hand, the PNIPAM segments are looser and more hydrated, adopt a restricted random coil conformation, and show a phase transition, which is dependent on both particle concentration and the chemical nature of the end groups of the PNIPAM chains. Monolayers of the amphiphilic gold nanoparticles at the air-water interface show several characteristic regions upon compression in a Langmuir trough at room temperature. These can be attributed to the polymer conformational transitions from a pancake to a brush. Also, the compression isotherms show temperature dependence due to the thermo-responsive properties of the tethered PNIPAM chains. The films were successfully deposited on substrates by Langmuir-Blodgett technique. The sessile drop contact angle measurements conducted on both sides of the monolayer deposited at room temperature reveal two slightly different contact angles, that may indicate phase separation between the tethered PNIPAM and PS chains on the gold core. The optical properties of amphiphilic gold nanoparticles were studied both in situ at the air-water interface and on the deposited films. The in situ SPR band of the monolayer shows a blue shift with compression, while a red shift with the deposition cycle occurs in the deposited films. The blue shift is compression-induced and closely related to the conformational change of the tethered PNIPAM chains, which may cause a decrease in the polarity of the local environment of the gold cores. The red shift in the deposited films is due to a weak interparticle coupling between adjacent particles. Temperature effects on the SPR band in both cases were also investigated. In the in situ case, at a constant surface pressure, an increase in temperature leads to a red shift in the SPR, likely due to the shrinking of the tethered PNIPAM chains, as well as to a slight decrease of the distance between the adjacent particles resulting in an increase in the interparticle coupling. However, in the case of the deposited films, the SPR band red-shifts with the deposition cycles more at a high temperature than at a low temperature. This is because the compressibility of the polymer coated gold nanoparticles at a high temperature leads to a smaller interparticle distance, resulting in an increase of the interparticle coupling in the deposited multilayers.